Microstructural Developments Through Marforming in a Ni-Ti-Fe Shape Memory Alloy
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FABRICATION of components, from flat and long products, is an important technological requirement.[1] This also has scientific and technological challenges. For example, when an ingot is finally transformed into a car door or panel, the challenges are to provide the desired shape with appropriate microstructure and properties.[1,2] This is the broad subject of thermo-mechanical processing (TMP).[1] The subject of TMP often involves hot working, followed by cold working and annealing.[1–3] Improved ductility and reduced flow stress are used during hot working, while the ability to maintain close dimensional tolerances plus the subsequent postdeformation softening holds the key to cold working and annealing. The mechanisms of plastic deformation, softening, and possible phase transformations allow suitable developments in microstructure–property and hence optimized performance of the final component. The science and technology of TMP is valid not only for large productions (examples: beverage cans to autobody)[1] but also for low-volume high-value components. Components made of shape memory alloy (SMA) can be a classical example. SMA components are typically fabricated using a combination of melting and hot and cold working.[4] The latter is often imposed in the martensite phase, the so-called marforming.[4–9] R. BASU, formerly Ph.D. Student with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India, is now with the Oxford Instruments India Pvt. Ltd., Mumbai, India. L. JAIN, Ph.D. Student, and I. SAMAJDAR, Professor, are with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay. Contact e-mail: [email protected] B.C. MAJI and M. KRISHNAN, Scientific Officers, are with the Bhabha Atomic Research Center, Mumbai, India. Manuscript submitted March 27, 2012. Article published online May 16, 2013 4310—VOLUME 44A, SEPTEMBER 2013
Though the open literature on SMA fabrication is ‘‘limited,’’[4] it is generally understood that hot deformation and/or marforming are followed by post-deformation annealing to get the desired microstructure and requisite properties.[5,7,8,10–16] Though hot working in the austenite phase, the so-called ‘‘ausforming’’,[4,9,11] often formulates the first stage of TMP, ‘‘marforming’’ has several advantages: accurate dimensional tolerances[6] and substantial grain size refinement.[7,8,11] A combination of marforming plus annealing can improve SMA performance: namely, mechanical behavior,[5,7,8,12] phase transformations,[5,8,12–14] and fatigue threshold.[10,11,15,16] Such property modifications need to be viewed in the framework of microstructural, including crystallographic texture and residual stresses, changes. There are some excellent studies on the crystallographic texture in SMA, but mostly on anisotropy and transformation strains[17–20] and rarely on marforming.[21] Paula and Braz Fernandes[21] have tried to relate marformed texture developments to the possible developments
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